Pulse code modulation communication system



March 15, 1949. J. R. PIERCE 294547607 PULSE CODE MODULATION COMMUNICATION SYSTEM Filed July 9, 1945 a Sheets-Sheet 1 BAL ANCED MODULATOR LOCAL OSCILLATOR BALANCED TRANS- HOMODYNE HOMODYNE DETECTOR 05150109 II? A D? B POL .4 R/TY PUL S E GENERATOR TRANSMITTER 4:76 2 LOCAL OSCILLATOR .X' l $/GNAL' -\Afi 6/ G2 l I F/GB F/GQ FIG/0 I HOMODY/VE DETECTOR 5. X7 FIG. 7

F/Gl/ F/e/z FIG/.3 xfij E 12221 7 LOW PAS! V OUTPUT F ILTE R PROPORT/OAML E TO M INVENTOR By J R PIERCE A TTORNE Y March 15, 1949. J. R. PIERCE 2,464,607

PULSE CODE MODULATION COMMUNICATION SYSTEM I 8 Sheets-Sheet 2 Filed July 9, 1.945

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PULSE CODE MODULATION COMMUNICATION SYSTEM Filed July 9, 1945 a Sheets-Sheet 3 FIG. 8

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PULSE-CODE MODULATION COMMUNICATION SYSTEM Filed July 9, 1945 a Sheets-Sheet 4 'J. c5 WWJQMZMW- ATTORNEK J. R, PIERCE 4 9 1 :w 1 an m Mav PULSE 001m MODULATION COMMUNICATION SYSTEM 8 Sheets-Sheet 6 Filed July 9, 1945 FIG.

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LOCAL OSCILLA TOR INVENTOR J R P(ERC ATTORNE V PULSE CODE MODULATION COMMUNICATION SYSTEM Filed "July 9, 1945 s Shets-Sheet 'r FIG. /2

TERM

INI/ENTOR 8y .1 R. PIERCE ATTOPNEK\ J. R. PIERCE 2,464,607

PULSE CODE MODULATION COMMUNICATION SYSTEM March 15, 1949.

Q Sheets-Sheet 8 Filed July 9, 1945 FIG. /3

INVE N TOR J PIERCE B um 1M.

ATTORNEY UNITE STATES PULSE CODE MODULATION COMB/[UNI- CATION SYSTEM John R. Pierce, Millburn, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application July 9, 1945, Serial No. 603,990

16 Claims.

This invention relates to communication systems for the transmission of complex wave forms of the type encountered in speech, music, sound, mechanical vibrations and picture transmission by means of code groups of a uniform number of impulses of a plurality of difierent types or signaling conditions transmitted at high speed.

An object of the present invention is to provide a communication system capable of transmitting and reproducing with high fidelity a complex wave form over an electrical transmission path in such a manner that the signal-to-noise ratio of the received signal is materially improved over the signal-to-noise ratio through the transmission path.

Another object of this invention is to provide improved and simplified methods and apparatus capable of transmitting and receiving signal impulses over a channel having a low signal-to-noise ratio and deriving therefrom signals having a high signal-to-noise ratio. In one sense this is obtained by using a wider band width, that is, trading band width for signal-to-noise ratio.

More specifically, it is an object of the present invention to provide methods of and circuits and apparatus for transmitting in succession a group of pulses in sequence over a given channel representative of the amplitude of a complex Wave at successive instants of time. The pulses to be transmitted are, in general, of short duration so that the duty cycle of the various elements of the system is low, thus permitting large momen tary power without overloading various elements, such large power tending to override such noise as may be inherently present, giving thereby a l more favorable signal-to-noise ratio. Also by such low duty cycle combined with suitable gating arrangements at the receiving station the amount of noise picked up at the receiver may be correspondingly reduced.

It is a further object of the present invention to provide improved apparatus for determining the code to be transmitted to represent each of a large number of different amplitudes without the use of complicated counting circuits,- arrangements and equipment.

A feature of the present invention relates to methods and apparatus for determining the magnitude of an electrical quantity and transmitting a series of pulses representative of said amplitude.

Another feature of the invention relates to methods and apparatus for building up an electrical quantity which is proportional to and'a measure of the amplitude of a sample of a complexadmittance.

wave at a given instant. This quantity may take on the nature of a conductance or an admittance.

Still another feature of this invention relates to methods and means to perform building up of this electrical quantity step by step by an additive or a subtractive process to a total which is proportional to the amplitude of a sample of a complex Wave.

Another feature of the invention relates to the use of conductances to be so built up by an additive or subtractive process to the total required.

Another feature relates to the transmission of information in the form of a code regarding each conductance added or subtracted.

This application is related to my copending application Serial No. 603,989 filed July 9, 1945, and its general objects and features are the same. It differs from the disclosure of that application in a novel method of introducing at the transmitter a group of conductances in parallel and deriving a voltage which is proportional to the introduced conductances and is compared with the sample voltage, being made equal to it within the degree of fidelity contemplated in the system.

Briefly, in accordance with the present invention, equipment is provided for generating a control pulse or a group of pulses of predetermined time relation one with another. These control pulses are employed to control a code element timing circuit, which circuit in turn generates a series of very short pulses some of which are posgenerates a series of code element timing pulses and these in combination with the sampling means derive an electrical quantity having a magnitude related to the magnitude of the complex wave at the time of the control pulse.

This electrical quantity takes on the character of an admittance which may be varied step by step, cooperating with the complex wave sample to determine the next step of variation in the More specifically, the electrical quantity is in the nature of a conductance made up of a plurality of conductances of different magnitudes which may be added to each other or omitted in such combinations as to yield a conductance related to the magnitude of the complex a smaller and smaller change is made. in the:

conductance, to see whether the potential dif.- ference is in excess of or below that of the signal, sample. If in excess of the signal potential difference, the last added conductance is removed; if below the signal potential difierence the added. conductance is left in.

In the specific illustration of my invention given herewith the: plan is followed that" if" the ma nit-udeof the conductance due to the last addition is in excess of that required; then it is removed and an on signal is transmitted. If 'theaddition is below that required, then it" remains-in and information to thisefiect-ispassed' tothe remote point bytheabsence of the transmitted signal, the identification of the information being specified by theti'meatwhich-the informational pulse istransmitted or not trans-- mitted.

Thisform of comparing the sample with the potential difference-set upby the conductance; is then carried on.step: by. step. to asfarza point.v as may bev desired. and in any case; to; an: extent so thatthegranularity ofthe signal finally reproduced ata; receiving point will be;within1 the. limltsyof fidelity contemplated: for: the y tem.

While thussettingup a'code of pulses com. pletely characterizing. the-amplitude oi: the com plex wave sample,- these pulses;- are transmitted. At the receiving stations. control pulse generator, code element, timing generator and a... system of conductances to be introduced or notwintroduced, all analogous to those of. the, transmittingstation are provided. In addition,. decodinsappae ratus. is, provided in which the received. pulses are employed to, produce an electrical quantity having a magnitude, similar to. the magnitude of the complex ,wavesample of the transmission end of thesystem andthus the complex wave, is reconstructed from a succession of such reproduced wave samples.

A featureof. this systemas a whole is thatthe measurement of the sample voltage is on a linear; basis, 1. e., the sum of the steps making up. theelectrical quantity is proportional to: the sample. voltage. In this respect the system is to be distinguished from ne in which. the measurementis on.a,decibel.basis, as in my copending appli cation, SerialNo. 592,961 filed May l0; 1,945,.

\ The invention itselfyboth as to-..its; organiza:. tion andmethod' of {operation togetherwitnotherobjects and features thereofwilrbe better-up;- derstood from the; following description: taken with the accompanying drawings in which:

Figs. 1 and 2 show-in functional block'formthe various elements and the mannerin which they cooperate to form an exemplary communication system embodying the present-invention;

Figs; 3"and 4 illustrate the timing and nature of" the pulses andwaves characteristic of my system;

Fig. 5 1s explanatory: of the method and" ap- J off, or 00010101.

paratus used for measuring or determining the amplitude of a complex wave sample;

Figs. 8 to 13, when positioned as shown in Figs. 6 and 7, give in detail the various circuits, equipment and operation of an exemplary system embodying the present invention; and

Eigr QAgis, explanatory of: a portion of Fig. 9.

Fig; 14 is a circuit diagram of acontrol portion of a conductance.

Referring more specifically to Fig. 1, let M be a: modulating; function representative of any complex: wave such as a speech Wave, a small portion of which is indicated by curve 35 of Fig: 3. A modulatonsends out a series of signals forth. Each series consists of 1+n signals of the onl*-off"" variety; (The signals need not be on croif but each signal distinguishes between two positions called on and off. These might be positions of time, frequency or amplitude. OI-1" might be transmitted as a current being of! and"oflf as that oi?- current being on.)

The-*fi'rst signal oftheseries or group is used to tell the polarity ofthe function M at the time tm. The next 12' signals of the group specify themagnitude of M at imwithrespect to some- These 11.

purposes); as to. represent, a-large number of differentamplitudes, Thus, for. the case of 8 channels theremay be sent; between tm and tm+1, the series of signals-on, on, on, off, on, oil, on,

The first zero carries, the information that, M. is. of. negative polarity and the. remaining. signals that, M is. of amplitude 21,times,the arbitrary unitvalueemployed, this value. being, obtained. by. binary counting. group. of pulses. of this type may be regarded as a permutated. series of. impulses in which the on" conditions. represent. marking selections and. the, oil conditions represent. spacing selectionsor-viceversa.

Ql'leespecific.meansv for obtaining such a seriesof signalsis. illustrated in the block diagram of; Fig.1. The over-all functioningwill be described;,-first andrthe possible nature of the block units ,willbe described later.

Whilethdfunctions describedcould be carried out without frequency conversion, grounding problems" are; simplified by first impressing the modulatingjunction Mon a balanced modulator I of-Fig. ,1, togetherwith the output of a local oscillator II or'frequency fa, high compared with the; frequencies occurring in M. At the time tm shown; on Fig; 3., a pulse which will be referred tOyflS fitll Mipulse, from a pulse generator VI is impressed on I. During the interval T between in. andtmn, when another impulse is impressed on- I,j themodulator; outputcurrent Im has a fre-- quency fe andanzamplitudeproportional to M atria.bcingz plusrwhen Miisplus at tm and minus when M is minusat I'm.

.intervalr'T the modulator output has the same frequency. foand the newamplitude. This is indicatedEin Fig. 3in which the graph 3&2 in

.thesecond line shows'the potential across a condenser (later to be described) and the graph BScihrjhQ lastgroupr shown in line. I, this is reso described in this specification for illustrative At tin-H another sample ofthe complex wave is taken and for the next aseaeor corded by a reversal of phase of the modulated current of frequency In.

Im is fed to a balanced transformer III. This and the local oscillator II are very low impedance sources.

Two homodyne detectors are fed with signals in opposite phase from the balanced transformer. In series with these is the drop across a resistance R caused by current flowing from terminals 0, d of the local oscillator II through conductances G1 Gn. This drop will be proportional to G1+ G11. In series also is an output from terminals d, e of local oscillator II. This is adju'sted to be equal and opposite to the drop across R' caused by the current flowing through G1 G11 in their minimum conductance condition. Thus the voltage across d, e taken in series with the drop across R is proportional to the changein G1+ G11.

A polarity and amplitude detector is shown at V. The two inputs A and B from the homodyne detectors IVA and IVB are shunted with biased diodes so that the voltage across their outputs can never become negative or positive by more than a certain fixed amount regardless of the input to the homodyne detectors.

At the start of the cycle of operation associated with any one sample of the complex wave, all of the conductances are set in minimum condition. One of the homodynes will have positive output, the other negative. A pulse is then applied to polarity and amplitude detector V. If the polarity of M is negative, an impulse is sent to the transmitter VII and this, perhaps in conjunction with a pulse from VI, causes the transmitter to send an on signal. If the polarity of M at tm is positive, V sends no pulse to VII and either VlI emits no signal, meaning off or a pulse from VI causes VII to send an off signal.

The conductances, G1 (in, are electrically controlled. Control of these from off to on" condition causes changes in conductance corre-- sponding to the digits of a binary number of the type mentioned. Thus, for the case previously given of l+n=8, the tota1 conductance is specifled by 7 digits. Turning the various Gs off and on in this case changes conductance by the amounts listed below:

Minimum conductance Gm Gu I Change caused by G1 64Go Change caused by G2 32Go Change caused by G3 16Go Change caused by G4 8G0 Change caused by G5 4G0 Change caused by G6 2G0 Change caused by G1 Go Thus the total conductance range is Go to 127Go in steps of Go. This makes possible a range of about 42 decibels with respect to the smallest step. The conductances G1 G11 are controlled electrically by (a) pulses from the pulse generator VI, and (b) the amplitude output of the polarity and amplitude detector V. The character and timing of these pulses are shown in Fig. 4 which is an expansion of the time interval between two sampling pulses or the time required for one cycle of operation. Here the second time interval T is shown expanded. The negative portion of the M pulse resets the modulator I and the positive portion immediately thereafter takes the sample of the complex wave. At the same time the initial negative pulse shown on lines I to n of Fig. 4 places all Gs in the minimum conductance condition. In the meantime the sample of the complex wave causes operation of the homedyne detectors IVA and IVB. Then a polarity pulse at a time indicated by the P line of Fig. 4 is applied to V and information on the polarity of M is transmitted. Following this, a plus pulse is applied to G1 at a time indicated by line I of Fig. 4. This puts in 64Go of conductance. Immediately thereafter a negative pulse is applied to G1. If the added drop across R due to the introduction of G1 is less than signal voltage, then the condition of G1 does not change and the conductance stays in. No impulse is sent to transmitter VII and either no signal is sent, indicating off, or a definite o signal is sent by a pulse from VI. If, however, the added drop across R exceeds signal voltage, then G1 is removed by the negative pulse and at the same time a pulse imparting that information is sent to the transmitter VII. This, perhaps in combination with a pulse from VI, causes VII to send an on signal.

Next, positive and negative pulses are applied in turn to G2 G11 at instants indicated by lines 2 to n of Fig. 4, resulting in insertion of conductance and transmission of off singals or in insertion and subsequent removal of conductance and transmission of the on signals.

After the 1+n signals specifying the polarity and the amplitude at he have been sent, VI sends a resetting pulse to the conductances. This puts all G's at minimum conductance condition. This pulse may also send a marker signal from the transmitter denoting the end of one interva1 or cycle of operation and the beginning of the next. This pulse may be simultaneous with the M pulse applied to I.

In a review of the operation of the system, reference may be made to Figs. 3 and 4, showing the character and time spacing of the pulses required from the pulse generator VI. At the top there is shown a series of pulses coming over the channel M to the modulator I and a sample of the complex signal wave at the instant of each modulator pulse is taken. Operating on an electrical element the sample causes the modulator to emit, until pulsed again, a current Im of frequency f0 and amplitude proportional to M at tm. All this is indicated in the second and third lines of Fig. 3. A polarity pulse to the polarity and amplitude detector V arrives an interval later and this is succeeded by pulses over the channels l, 2 n which operate on the conductances G1, G2 G11. There is also a pulse channel T to the transmitter, necessary if the ofi signals are transmissions and not merely omissions. All this is shown in Fig. 4 which is an expansion on a time basis of one of the sampling periods of the system.

At the time of the M pulse, channels i and n are pulsed with strong negative pulses setting G1 Gn in the minimum conductance conditions. Next the polarity and amplitude detector V is pulsed. This results in a pulse to the transmitter VII if the polarity is negative and an on signal, or no pulse to VII if the polarity is positive and an off signal.

The n amplitude channels are pulses in sequence, first positively and then negatively. The positive pulse inserts conductance, the negative pulse removes conductance if it is too large and in doing so sends an on signal, otherwise the conductance stays in and there is an off" signal. The channel T may apply pulses to the transmitter VII at all times whether an on,

an -o i. or a marker?-signalistotbe-sent: These pulses will'givean ofif' signal from'the transmitter but in combination with a pulse from (H Ga or 'V may giveanon? signal. Channel. T. may be omitted-in.-Which case off signals will be-omissions.

While a number of diiferent types of devices may be contrived to perform the functions of the representative.blocksof Figs. 1 and 2, a better understanding. of the invention will be -obtained by. specific illustrations of circuits to -ac complishthe desired. ends. These will now be described but it is to be understood that they are illustrative only andth'at my invention is not limited to the specific circuit arrangement shown.

Transmitter Fora description of the transmitting end of my'system, reference should be made to Figs. 8, 9 and 10 in which the various block components of Fig. 1 are represented by the same Roman numerals.

Thecomplex .wave transmitted is impressed on the1balanced .modulator system I, this complex wave having come-from any suitable source such as.a microphone 805 through appropriate terminal equipment 800. It is then sampled periodically and goes through the process to be described in further detail.

Pulse generator VI It will be advantageous to now describe the pulse generating system VI, one form of which is shown in detail in Fig. 10. The first controlling element in this portion of the system is a relaxation oscillator comprising agas-tube mm. This relaxation oscillator is of a form well known in the art and includes a resistance IIJEI for charging a condenser IOI2. Assuming that to start with the condenser IOI2 is discharged then on closure of the circuit it is charged at a rate determined by the resistance 50. potential of the condenser and consequently the plate of tube I0! 0 rises to a firing value, the condenser suddenly discharges through the tube and resistor IBM. The duration of the discharge is short and gives rise to a sharp positive pulse across the resistor IBM. The duration of this pulse and'the rate at which it is followed by'identical pulses, can be completely controlled bythe parameters of the circuit; in particular, by the values of the elements Hill to IOMtaken with the potential of the grid of tube IOI0 as determined by the potentiometer IOI5. While any of'several forms of relaxation circuits may be used at this point the one shown is simple and satisfactory. Its operation is more fully described in many places, suchas on page 184 of Ultra-High Frequency Technique by Brainerd et-.al., published by Van Nostrand Company, 1942.

When the The'positive. pulse from |0I4 is. now'used to 4 control the emission'of pulses: to various parts This is accomplishedby con-- of the circuit.- necting across the element I DI 4 a delay circuit IOI6 made up of identical sections of inductance.

and.capacitance-connected seriatim. A positive pulse travels through this network, the time of arrival at each section being uniformly spaced and giving rise to corresponding positive pulses going out over channels I, 2 n forpurposes to be described. The delay network is terminated by a load I040 of proper value to suppress any reflected Wave.

The parameters of therelaxation oscillator maybe adjusted so that pulses are derivedacross resistance; IOI4r attanygfrequency desired; For. the purposes of my invention.it;is-.preferred...to;. have a samplingfreqllency; higherthan that of the highestfrequencycomponent in the complex.

.:able value, therefore, for the relaxation'oscilla-torfrequency WOllld'bG 8,000 cycles although. a higher value may beused, if desired.

By meansof the delay circuit'or timestiok? I0l6, one has available at the ends oftherespec-e tive sections, positive! pulses similar to that initiated in IBM andspaced in time, one after the; otherbyan intervaldetermined by the elements in a-section of the timestiok. At time in wheni. the pulse is formed at H4 (or point M) it'is; transferred immediately :to tube I02 0- and iszthen transferred to I asthe M pulseof Fig. 3.. The function of, this pulse will be described herein after.-

At oneelement of time later the positive pulse.- from I 0I4- will have reached the point P on the timestiok and this will be identified as the P. pulse. This P pulse operates on the grid of tube I02I -to give a positive. pulse over the resistor. I 022 which pulse is transmitted to the polarity," and amplitude detector: V, appearing there asa. positive pulse ofa vform and at a-time indicated.- by line P of Fig. 4.

At the end ofsucceeding elements of time the positive pulse from I0.I'4 will arriveat pointsI 2 n of the timestiok and then be transferred. respectively to. the grids of tubes I08I, I082, I083.

etc. Each of these-pulses in its--subsequent:path.-

is. converted into a positive-negative pulse-in a: manner and for. the purpose hereinafter de.- scribed.

In addition to these. pulses it is desired to send a group of corresponding pulses to the transmit-- ter VII .which pulses will be. identified as T pulses and are indicated in the bottom line of Fig.4. It will be'noted that the. first pulse in this cycle:

is longer than the subsequent pulses, this for.

reasons which will appearlater. The first T pulse may, for instance, be approximately twice the length of the subsequent pulses. For the formation of the T pulses a chain of'tubes I024'to I025 is provided. The grid of tube I024'is operated on directly by the pulse from M. In the cathode circuit there is included the resistance I03I-paralleled by the condenser I032. A positive pulse,-

is'generated across I03l and the duration of this.

pulse would ordinarily be-the'same as the duration of the -M pulse. However, the additionof "a condenser I032-will lengthen this pulseand the condenser is so chosenas to approximately double.- the duration. The positive pulse from I03I.-is applied to the grid of I025, the cathode circuit of which. includes. the resistor I030. At the time-.- of. theM' pu1se-onlthetimestiok there is then: transmitted over. the T channelv to the trans?- mitterVII a positivepulse of approximately twice:

the duration of the M pulse. As thepositivepuiser over. IOI4 reaches; successively the points. P,- I, 2 7? the gridsof the tubes, l020to I029: are operated upon and set up positive pulses; acrossresistor I030 which is a commoncathode; resistor for all the tubes I025 to I029. Consequently there is transmitted the desired; series of: positivev pulses to'the transmitter'VII indicatedin Figs. ,4 and. 8. For the purposeofsynchronizing.

.-. these; pulses; with the; remainder of the; equip:-v

ment, delay circuits may be introduced wherever necessary. One such delay circuit is shown in the T channel at I050.

Following the M pulse and the P pulse the number of sections in the timestick will usually be made equal to the number of digits required for setting up the amplitude code to be transmitted from VII. If there should be n of these then the number of possible codes by permutation of on and ofi signals would be 2. Thus, if number of digits in the code is seven this will make possible 128 combinations so that in the system it will be possible to discriminate between amplitudes of 128 different values.

Associated with conductor I from the timestick and tube IOBI is a circuit comprising tube I06I and transformer I'II'. The tube I06I is shown as a double triode. The grid of the left-hand section of this tube receives a positive pulse at tm which is then converted at the plate to a negative pulse. This negative pulse is transferred through transformer Hill as a negative pulse to the control of the corresponding conductance, device G1 in Fig. 9 and serves as hereinafter described to set this conductance element to a minimum conductance. Shortly thereafter a positive pulse arriving over conductor I to tube I08I is inverted and appears as a negative pulse on the grid of the right-hand section of tube I 06L However, the load circuit of tube I08I includes the inductance I09I which causes the negative pulse generated on the plate of I08I to be immediately followed by a positive pulse 50 that the pulse arriving on the grid of the right-hand section of I06I is a negative-positive pulse. This pulse in turn is inverted by the right-hand section of tube I06I to a positive-negative pulse which is then transmitted through the transformer I'0'II to the control circuit of conductance G1. The character and timing of this positive-negative pulse is that indicated in line I of Fig. 4.

A similar circuit is associated with each of the conductors 2, 3 n to give at the time of the M pulse a negative pulse to the corresponding conductance control devices setting each conductance to a minimum conductance and at a later time transmitting a positive-negative pulse sponding transformers and to suppress their differentiating action, thus preventing the setting up of a reverse pulse. Other elements all serve purposes well known to those skilled in the art and need not be described further.

Modulator I A description will now be given of the modu lator circuit I as shown in Fig. 8. A complex Wave made up of numerous frequency components, the highest one of interest being for the present taken as 4,000 cycles, arrives at the primary of transformer 8I0. At time tma positive pulse arriving,

on the grid of tube I020 is converted by trans former ISIS and is impressed as a negative-positive pulse on the plate and cathode of diodes 8H and 8I2. The negative pulse causes current to gflow through 8I2 and discharges condenser 8M.

When the positive pulse arrives immediately thereafter it causes current to flow through 8 charging the condenser 8H3 to a definite potential, this potential being equal to the positive pulse plus that of the amplitude M at tm minus a constant bias potential determined by battery M0. The grid of triode 820 is held at this potential minus a bias potential due to 82I until another pulse is applied at tm+1. The output of the amplifier tube 820 appears as a voltage across the resistance 825 and controls the operation of a conventional balanced modulator of any suitable form. As here shown it involves two varistors v1 and 02. Associated with this balanced modulator is also a source of local oscillations II of frequency in large compared to the pulse frequencies present in thesystem. In a manner well understood in the art there will appear in the secondary of transformer 830 a wave of carrier frequency in, the amplitude of which will be proportional to the potential across resistor 825. The phase of the voltage in the secondary of 830 reverses as the voltage across 825 reverses. The output of the secondary of 830 is connected to the grid circuit of a pentode 835 which in turn yields an output current Im which is almost independent of the load consisting of tuned circuit 831 and associated elements appearing hereinafter.

Local oscillator II The local oscillator II may be of any one of the suitable forms well known in the art yielding a substantially sinusoidal output of reasonably constant frequency.

Homodyne detector IV Associated with the output of the pentode 835 is the transformer 90I which connects to two homodyne detectors IVA and IVB in a manner now to be described. The primaries of two transformers 903 and 904 are connected in series and transfer the modulated carrier of frequency in to the input of amplifying tubes 905 and 900, the amplitude of this carrier being determined by the amplitude of the signal sample. A bridge circuit from the mid-point of 90I to the common point of 903 and 904 includes certain elements described below. The output of tubes 905 and 906 through transformers 901 and 908 is supplied to the two similar balanced demodulators IVA and IVB. These demodulators may take on any of the wellknown forms. Here they are shown as each including a pair of varistors and being supplied with a carrier of frequency fo from the local oscillator through transformer 909. As a result, there will appear across the output resistors 9H and 9I2 a potential difference of pulse frequency,

As an illustration of the operation of the system, let it be assumed that at a certain instant the current in the secondary of QM is in the direction indicated by the arrow, giving rise to currents as indicated in primaries of transformers 903 and 904. It will be assumed also that the poling of the transformers from the local oscillator is such for positive sample amplitudes as to make the terminal is plus with respect to l and the terminal m minus with respect to l. The symmetry of the system is such that the potential differences across 9 and 9I2 will be equal so far as input from transformer I alone is concerned and, during any one sampling period, constant and proportional to the same amplitude. o

It is the purpose in this circuit to obtain a measurement of this sample amplitude and to this end I make useof a variable conductance.

' purpose comprises a circuit connected from the mid-point of the secondary of 9! to the common point of the primaries of transformers 903 and 904. Included in this branch is the secondary of a transformer-9|? supplied from the local oscillator. A portion of this secondary supplies current through the conductance G in series with a.

resistance R and there will appear across the res'istance' R a potential drop due to the local oscillator frequency current flowing therein. The one end of resistor R'is connected to an intermediate point of the secondary 9i 8, this point being so selected that when the conductance G (9 i 6) is at'its .mlnimum value'the drop over R will be equal and .oppoSite to the electromotive force across the upper portion of M8. 'Thus the potential difference between fand g is equal to zero when thee conductance-G is at minimum. 'As noted here- "tofore; thetransformersfillll and 9l1an'd associated tubes supplyinr-f'them are designed to act as very low'impedance sources. If, also, the

resistance R is small compared with the recipro-l cal of G at any of itsvalues, then it can be shown that the change in dropover R and therefore the potential difference between 1 and y will be substantially proportional to the conductance G.

This potential difference-andits direction is indi- 3 -catedat anyone instant by e and by the downward pointing arrow.

When e is equal to zero, corresponding to minimum conductance,'--the potential differences between k, Z and between I, mwill be equal.

however, theconductance is increased so that e takes onadefinite value then, for the'directions indicated, it will be apparent'thatthe current flowingthrough the primary of "903' will be reduced and that through 904 will'be increased. Accordingly, the potential at '70 will become less positive with respect to the point I and the poten- -tial at in will become more negative with respect "to Z. If the polarity of'the signal sample should 'reversewthen the. polaritiesacross 9H and 9|2 will also reverse and this reversal may be used to indicate the polarity of -the signal sample in a -manner hereinafterdescribed. Furthermore, the change inpotential of'the points k'and ml may be used to determinethe amplitude of the sample,

this being'accomplished by so adjusting the value zero.

Polarity and amplitude detector V pulse from tube" I92 l of; Flg.'10. Thistransformer tends to put a positive pulse on the grid of-trio'de former 93B.

934 but because of the potential differencefrom 2 tom and additional bias this tube'is'cut ofi when the sample amplitudeis positive and thus no pulse is transmitted through the output-trans- If, however, the polarity of'the sample amplitudereverses; sothat the potential difference over Z, "m reverses. thenthe'pulsein transformer 935 renders the triode' 934 conducting' and a pulse is transmitted over' transformer 936 to the transmitterVII; giving the informa- .'-ductance' circuits. G1

control circuits C1 tion of negative polarity. The operation of 'the amplitude measuringfeaturewill be described below.

Conductance III *In' Fig. 9 there'areshown a plurality of con- Cm, and conductance Cn,-'one for eachdigit in the-amplitude code. These 'conductances-taken in parallel, in whatever combination desired, constitute the conductance G symbolically repre- -sented at 9l6. Three such units are shown but inasmuch as their action is identically-the same except for timing and conductance value, it is necessary to describe 'only one 'of these identified bythe-b1ock III=which includes the first conductance G1 of the-series and its controlling unit/C1.

-The conductance circuit is essentially a' shunt 'feedback amplifier. 'It includes the resistances R1 and R'i connected in" series the intermediate point being connected-t0 the three-stage amplifier-including the tubes*94|,' 942,943 with resistance capacitance coupling; there'being a feedback connection from the plate of the last tube :943 through condenser 944*to the grid'of-94l.

There is either zero ora very large gain-around the loop depending on whether the control' 'v'o'ltage applied to terminalsa and 12 cuts oif 'one or more tubes in the loop or allows them to operate. In this instance the'loop is opened by a sum- 7 ciently high negative bias on the grid of tube 942. Under these conditions the conductance'Gi is essentially If a positive potential of sufiicient magnitude arrives'at the'point b then the loop is closed and "if the gain through-the tubeis high 3/1 is virtuallyshort-circuitd and Gris essentially --almost independent of gain. "Thus a highaccuracy -'of control of Gr-can. be attained independent of'tube characteristics. "Whereas for illustrative purposes the control has been shown on one 'tube'only,"itmay be desirable to controlthe The circuit arrangement fondetei'mining the v polarity and amplitude is shown at V. Connected j across the terminals 7c and m is the resistance 93! the mid-point q of which will take on a potential bias on several'or'all 'tubesinorder to open the loop completely.

The control circuit 01 comprises a plurality of diodes -'95l, 952, 953 and-associated elements.

"fwhen a 'positivepulse over channel Z, arriving throug'h'tube I081 ofFig.--10, is applied, the diode 95I conducts-, charging the condenser 954, apply- 'ing' apo'sitive potential to b-and closing the feed- 'l-his will be referred to as introducing G1. ':Im-

-mediately' thercafter;anegative pulse is applied "back loop thus increasing the conductance G1.

through -transformer* 955 to diode 952. 'If this is the only pulse present =on '952 that is, if -lnisin'cludedthe transformer e'aswmeh receives a 75 sufilcientpositivebias is present from the amplitude detector V, the negative pulse discharges condenser 954 and opens the feedback loop thus removing the conductance. At the same time current flows in resistance 958 through diode 953 sending a negative pulse to the transmitter, resulting in the transmission of an on signal. If there is a positive bias of sufficient magnitude from the amplitude detector through diode 931 to be described later, the negative pulse through transformer 955 is not sufiicient to cause current to flow through diode 952. Consequently, the conductance remains in and no pulse is sent to the transmitter.

With the arrival of the next pulse, corresponding to channel 2 of the timestick, the sample voltage in transformer 90! is again tested by the introduction of conductance G2, and its removal if necessary. Similar action follows for each of the remaining conductances.

The operation of polarity and amplitude detector V for testing or measuring the amplitude will be better understood by reference to Fig. 9A. If the potential difference e is equal to zero, corresponding to G at minimum, the potential It would be positive with respect to Z by say Vi;

m would be negative by the same amount Vz so that the point q on resistor 93l would be at the potential of Z. However, the limiting diodes 932 and 933 will not allow the potential of m to fall lower than a designated value V2, or the potential of k to rise above an equal value V1 and the potential of q will still be the same as the potential of the point Z, say V0. When G1 is introduced the potential V'2 would tend to become lower but is still limited to V2. also tends to become lower and is still limited to V1. If sufficient conductance has been added so that e is slightly larger than the voltage in the lower half of the secondary of 9N, wlzenfithe polarity of 9H reverses and 7c becomes negative with respect to Z, attaining a value as low as V3 which is the same as the potential V2 at m; in other words, both 70 and Z take on the same negative value and Vq also falls to this value originat-,

ing a negative pulse from q. Connected to the point q is a circuit comprising the diode 93'! and battery 938 which normally is sending a current to each of the resistors 956, 916, etc., in the control circuits in such a direction as to make the upper ends of these resistors positive, thus giving a positive bias to the cathodes of 952, etc. 'With this drop across 955, the negative pulse alone over channel I is not able to make diode 952 conducting and therefore condenser 954 is not discharged and the conductance G1 remains in. When the voltage e exceeds the signal voltage, the negative pulse at q renders diode 931 non-conducting which is equivalent to sending a negative pulse to resistance 956. In that event diode 952 becomes conducting, condenser 954 is discharged and conductance G1 is removed. Thus it is seen that if the conductance introduced does not give rise to a sufiiciently large voltage e, the conductance remains in, but if the conductance in-.

troduced is such that e exceeds signal voltage, then the conductance is first introduced and is then removed by the negative pulse at q. It will be noted that if the polarity of the signal at 9M reverses then the polarity of k and m also reverse. In this case the voltage 6 will reduce the current in 904 instead of 903, thus reducing the potential of m. The potential of It tends to become more negative but is limited. When the cross-over point is passed the potential of 111. goes negative and q drops to the same negative value The potential V1 as in the first instance. Thus, q goes negative for excess value of e for either polarity.

With each succeeding amplitude code pulse the successive conductances G2 Gn are introduced and. are allowed to remain or are removed. At the end of the cycle, the total amount of conductance introduced is that which will yield a voltage e as nearly as possible equal to that of sample voltage in transformer 99 I.

If a conductance is introduced and remains in then no signal, corresponding to an off signal, is sent. If it is removed by the discharge of condenser 954, current flows through resistor 958 setting up a potential difierence which renders diode 953 conducting thus sending an on pulse to transmitter VII.

During the setting up of the conductances, pulses modulated on a suitable carrier will have been transmitted from VII bearing the information to the remote station on what conductances are being introduced. The amplitude of each of the pulses so transmitted from VII will be the same and each element of the signal is purely an oil? and on matter. Since only integers are sent such a signal can be repeated indefinitely without adding distortion or noise to the recovered intelligence, even though distortion and noise below a certain threshold level may be present in the repeaters. Thus, even for very high quality transmission the requirements on the repeater units are low. This makes possible transmission over long paths with many repeaters. The presence or introduction of noise in the transmission path from VII to the remote receiving station will be of no influence so long as the noise thus introduced is relatively small compared to the amplitude of the signal being transmitted over the path. Such noise, therefore, would not appear in the signal later reproduced.

The value of the various conductances is more or less arbitrary but, in general, it would be expected that the first conductance G1 would be the largest, the remaining ones decreasing sequentially. Thus G1 may be of a value such that the voltage e takes on a value of 64 units of potential difference, G2 of such a value as to introduce 32 units and so for the succeeding ones as indicated on column 5 of the specification. If two or more conductors are introduced the value of e will be equal to the sum of the effects of these conductances taken individually and thus the total value of 6 can be made equal to the sample voltage,

within a limit represented by the change due to the smallest available conductance Go.

The action is made more clear by reference to Fig. 5 in which, exclusive of the polarity pulse, a five-unit code is to be used on a binary system. If, for illustration, the sample amplitude is slightly over 22 units on the scale of 32, as shown at the left of Fig. 5, then G1 is such a conductance that its introduction by the pulse over channel I results in a change in voltage e of 16 units. This is less than the sample Voltage and therefore G1 remains in and an off pulse is transmitted to the distance station. The positive pulse over channel 2 now introduces G2 which adds another eight units to the voltage e giving a total of twenty-four conductance units and a total of twenty-four voltage units. This, however, is in excess of the sample voltage and therefore G2 is removed, as indicated on Fig. 5 and an on pulse transmitted to the distant station. The pulse over channel 3 then introduces G3 giving e a value of twenty units and this being less than the sample .a coaxial cable, wave guide, etc.

voltage .it remains in -so the .No. .3 :pulse is. an

off pulse. Thepulse overchannel ,4: nowintroduces G4 building 6 up .to twenty-two units.

Finally, thepulse over channel Bintroduces-Gs building e to 23. Thisis'in.excessof-thesample I a 'voltage and therefore G3 is removed. Thus voltage e has beenbuilt up which isasnearly equal to the sample voltage across one-.half-of the secondary :of 9D has iscontemplatedin. the sys- ;tem.

The marginal voltage between V1 and V2 .of

:FigHQA can be adjusted to as small :a valueas .desired, this-being controlled mainly by the bias- -.ing.batteries of the diode limiters 932 and 933. In general, it should be adjusted to be appreciably less than that corresponding to the voltage-developedby the smallest change Gain .con- .ductance, as this voltage appears at IE0? or 908 .after amplifications.

Transmitter unit VII During this procedure therehas been arriving attransforiner 856 a seriesof pulses over channel T,.one for each pulse'from the pulse generator, .timed as indicated on the bottom'line of Fig. 4. These pulses may be used to. operate ona grid of .the tube 855. In addition,'there arrives at'the transformer 852 certain pulses, one "for each on pulse, relating to polarity or indicating that oneof .the conductances has been introduced and then .removed. No pulse will come to 'the'transformer .852 if a conductance has been introduced'but'inot removed, this corresponding to anoif signal.

The secondary of the transformer 852 may operate on a second grid of tube '855 and this tube :in turn controls the transmission or absence of transmission over a suitable medium to a remote station. In Fig. 8 it is shown .as controlling a transmitting terminal unit'BBll for a radiochannel on a suitable carrier including carriers having wavelengths in the microwave region wherethe Waves have quasioptical properties of propagation. Also it is to be 'understood'that .thepulses coming from the tube 855may go'directly'to'any suitable transmission path such as apairof wires, "In such cases .it is.not necessary and may not be desirable to use the pulses for modulating a carrier. The connection of the transformers 850 and .852 are such that apulse arriving at 850 alone will not cause the transmission of a signal but the simultaneous presence of a pulse on 850 and on 852 would be effective in causing such transmission and would correspond to an on signal. The

purpose of the pulses coming over the T channel I .is to assure proper timing coordination of the transmission of signal pulses. In some instances such added precaution will not .be necessary in which case the 'T channel may be omitted, including the chain of tubes H124 to (029 of Fig. '10 and the transformer 850. In this case also the adjustment of the transformer 852 and tube 855 .is such that a pulse on 852 will then be sufficient to cause transmission.

Receiver A lo v .of the low-pass filter.

:16 .tions .coordinateproperl-y .with the incoming sig- ;=nals. Thisgenerator sends out pulses to various .deviceswhich also receive .thesignal. Thesere- :ceived signal.pulsesalonecause operation. A sig- .nal pulsein conjunction with a pulsefrom VIII will prevent .a device from operating.

At the time when the signal corresponding .to polarity appears the pulse generator applies .a P vi-pulseto aphase shifter IX. If thereis an ofi s signal or no signal .pulse the phase shifter isset ...to shift phase 180 degrees; .if an on signal the phase shifter is .set .to .shift phase .zero degrees. The phase shifter remains in this position until zreceiving another -P ,pulse.

.Thenext .pulsebf the pulse generatoris-sent ,.toian.electricallycontrolled conductance G1 which .controls a change of conductance proportional to thatof Gi of Fig. 1. The conductance is initially at a minimum. .Ifthereceived signal is an off ..signal, conductanceis switched in. If an "on :signalis received .this prevents operation and G1 is left .at minimum conductance position. The .nextpulse from VIII goes to G2 at the same time that .thesignal from Geof Fig. 1 arrives andso on through Gn. Thus, the total conductance is ..made proportional .to that introduced in Fig. 1 which gave rise to the on and off signals. lllhen .at .a short time later, perhaps the next .marker pulse corresponding to time tm+1 at the .sender, a pulse from the pulse generator enables the local oscillator X. This .acts as a constant voltage source of desired frequency sending cur- ;rent through G1 Ga .and resistance R and .to lthehom0dynedetector XI. R. is made small compared to (G1+Gz+ Gn) From the voltage .drop across Rthere is produced an output pulse from the homodyne detector XI nearly proportional to .the amplitude of M at tm. A reset pulse :mayfollow the pulse to oscillator X. This will re- ;set.G.1 .Gnand phaseshifter IX to the initial position. The pulses .from the detector XI are gpassed'throug ha low-pass filter XII. If the highest frequency fm in'the complex wave is fm=, /2 T ;and the low-pass .filter has a cut-off of jm, .a signal proportional to M, but delayed perfhaps by .T seconds, is recovered at the output fiHere T is the length of .a period between him and vtm+1.

With .thisbrif description of the block diagram .of Fig. .2 we may .now proceed to a more detailed description of devices .which will accomplish the steps setpforth above. It will be apparent to those skilled in the art that there are numerous .circu-it arrangements .for accomplishing this. .Certain specific circuit arrangements are here -s'hown in Figs. 11, 1'2 and .13 :but these are illus- .tr-ative .for the sake of concreteness and it is to .be understood that many variations may be made without departing from the spirit of my invention.

Referring more specifically to Fig. 11 there is shown a receiving unit 106, here indicated as a -radio .receiVer associated with a suitable receivzingantenna H05. This unit N06 is an entirely .conven'tional radio receiver of any suitable type .including a detector the output of which yields .the pulse signals as a reproduction of the pulses -=.arriiving at the transmitting unit VII of Fig. 8. This code pulse message .is amplified to any necessary extent as illustrated by the tubes H08 and .ljllllland the output of 3| I09 is shown as going to a plurality of control devices C1 Cn, one associated with each of a plurality of conductances =G1 G11, in a manner hereinafter to described.

Receiver pulse generator VIII A derived path from the output of H08 passes through suit-able amplifiers as shown at III2 and III4 and an output pulse therefrom is used to control a relaxation oscillator shown in Fig. 13. This relaxation oscillator, centering about the gas tube I3II1, may be simpliar in every respect to the relaxation oscillator at the transmitting station and shown in detail in Fig. 10. Corresponding to each of the units IDIIl to IOI5 in Fig. there are the units I3I0 to I3I5 in Fig. 13. The adjustment of the parameters in the relaxation oscillator of Fig. 13, however, is such that the circuit does not normally oscillate but is triggered off by a pulse arriving from tube III4. Furthermore, the parameters of this relaxation oscillator are so adjusted that the circuit will be triggered ofi by the first pulse in a group (correspording to the M pulse at the transmitter) after which the cscillator cannot be triggered oil until the arrival of the next M pulse.

This is accomplished by the use of the long initial T pulse of Fig. 4 to which reference has been made. It is to be borne in mind that all pulses transmitted by VII are of the same amplitude. However. since the tube III2 is essentially a constant current device, the voltage built up in the tank circuit I H3 is proportional to the duration of the incoming pulse and thus the pulses arriving at I3I0 corresponding to the M pulses will be of greater, perhaps double, amplitude and so be able to trigger the relaxation oscillatir, whereas the other pulses in the cycle will not.

In a manner analogous to that of Fig. 10 there is associated with the relaxation oscillator a timestick I3I6 from which a series of pulses may be derived with a time spacing as nearly identical as may be necessar to the time spacing of the pulses derived from the timestick at the transmitting station. This timestick has an additional section given rise to a pulse indicated by n and delayed only slightly behind the previous pulse. The function of the pulse 12 will be given hereinafter. The timestick is terminated with a suitable impedance I318 to suppress reflection. Also there is a series of tubes I325 to I329 from which a series of positive pulses derived from cathode followers is initiated corresponding to the pulses from the timestick.

Receiver conductances XIII The utilization of the various pulses to control the setting up of a series of conductances G Go corresponding to those set up at the transmitting station will now be described. Fig. 12 shows a plurality of conductances G1 Gn which may be identical with those at the transmitting station or may be proportional to them. Here again three such circuits are shown but inasmuch as their action is identically the same expect for timing, it is necessary to describe only one of these identified by the block XIII including the first conductance G1 of the series and its controlling circuit.

The conductance circuit is again essentially a shunt feedback amplifier. It includes the resistances R1 and R'i connected in series, the intermediate point being connected to the threestage amplifier including the tubes I261, I242, I 243 with resistance capacitance coupling. there being a feedback connection from the plate of the last tube I243 through condenser I244 to the grid of I24I. There is either zero or a large gain around the loop depending on whether the 18 control voltage applied to terminals a and b cuts off one or more tubes in the loop or allows them to operate. In this instance, as in Fig. 9, the loop is held open by a sufiiciently high negative bias on the grid of tube I242. Under these conditions the conductance G1 is essentially If a positive pulse of sufficient magnitude arrives at b then the loop is closed and if the gain through the circuit is high, G1 is essentially or more diodes or combination of diodes and triodes or multigrid tubes in a variety of ways as will be clear to those skilled in the art. Specifically, in Fig. 12 it is shown as a combination of a diode with a triode. A positive pulse corresponding to the M pulse from the timestick opcrates through the transformer I252 so poled as to make the cathode of I25I negative whereupon any positive charge on the condenser I254 is discharged and negative bias on the tube I242 opens the conductance loop. This occurs simultaneously on all of the conductance units at the beginning of a cycle and sets all the conductances at minimum conductance.

In due course a pulse from circuit I of the timestick and tube I321 arrives at transformer I256 being so poled as to make the grid of triode I251 positive, whereupon current flows through the plate circuit to give a positive charge to the upper plate of condenser I254 thus closing the conductance loop. This is on the assumption that the only pulse acting on the tube I251 is that coming from the transformer I256 and this would be the condition for an off signal arriving at the receiver. If, however, there has arrived an on signal at the same instant then that signal operates through transformer I258 in the plate circuit of I251, and when properly poled will balance the effect of the positive pulse on the grid and no plate current will flow. Consequently, condenser I 254 will receive no charge and the conductance loop will remain open. Thus'it is seen that conductance G1 will be introduced if the corresponding conductance G1 has been introduced at the transmitting station and if it has not been introduced at the transmitting station it will not be introduced at the receiving end.

The same operation will take place for each of the conductances G2 Gn, after which the combination of conductances connected in circuit will be the same as that at the transmitting end.

Local oscillator X The receiving station is provided with a local oscillator X shown in Fig. 11 as I I20. This may, but need not, be of the same frequency as the local oscillator II at the transmitting station. In other words, no synchronism between the two oscillators is required.

ance control circuits.

19 Phase shifter IX Phase shifter IX is shown as comprising two triodes I22I and I222 the grid circuits of which are supplied in parallel from the local oscillator H20 through the transformer I223. The output circuit comprises transformer I224, the midpoint of the primary of which is connected to the positive terminal of the plate battery. The grid circuit of tube I22I contains the condenser I225 and the grid circuit of tube I222 includes the battery I226 which tends to give a positive bias to the grid. Normally, therefore, the transconductance of tube I222 will be higher than that of I22I and there will be an alternating current of local oscillator frequency in the secondary of I224. The phase of the current in the secondary current may, however, be reversed by means of the phase shifter. For this purpose there is included two diodes I23l and I232 biased so that normally they are non-conducting. When a pulse from the tube I326 corresponding to P pulse arrives at transformer I232 it is so poled as to render diode I23I conducting, giving a positive charge to condenser I225 of such magnitude as to give tube I22! a higher transconductance than I222, whereupon the current in the secondary of I224 is reversed in phase. This reversal occurs if the pulse code at the transmitter was an off" signal, meaning an absence of a received pulse. The condenser I225 is so connected as to retain its charge for the duration of one complete cycle.

If, however, the polarity of the sample of the complex wave at the transmitter had been negative, then an on P pulse will have been transmitted and, in turn, received at the receiving station. A corresponding pulse, therefore, arrives at the transformer I234 in parallel with the transformers I258, etc., at each of the conduct- The transformer I234 is so poled that its pulse opposes that of the pulse coming on transformer I232 and consequently diode I 23I does not conduct, the condenser I225 does not become charged and the phase ofthe oscillations in secondary of I224 is not reversed. At the end of the cycle or the beginning of the next cycle the M pulse from tube I325 operates through transformer I236 to make diode I232 conducting, whereupon the condenser I225 is discharged or reset to normal condition. Through the means thus described it is seen that it is possible to change the phase of the local oscillator current in transformer I224 by 180 degrees.

The secondary of transformer I224 is connected in series with a resistance I2I6 corresponding to R of Fig. 2. In series with this also is the combination of conductances G1 Gn. Resistance I2I6 is small compared to the resistance of the conductances taken in parallel. Also the tubes I22I and I222 should appear as a low impedance source by any suitable means, such as using tubes of low impedance, or by use of a cathode follower circuit or by a step-down transformer. In this case the current in and the voltage across I2I6 'will be proportional to the conductances which have been introduced and, therefore, proportional to the sample current or voltage at the transmitter.

H omodyne detector The homodyne detector XI comprises a. balanced demodulator including varistors I2 and I2 I2 connected in a standard bridge circuit. The demodulator is supplied directly with local oscillator frequency through transformer I2I3 and also with the same frequency through transformer I2I4. The primary of transformer I2I4 is included in the plate circuit of triode I2I5, the grid circuit of which includes the resistance I2I6. Thus the detected current of pulse frequency appearing across resistance I2II will be proportional to the current in I2 I4 and therefore proportional to the amplitude of the complex wave sample. Furthermore, it polarity will be determined through the phase shifter IX to correspond with the polarity of the complexwave sample at the transmitter.

The last step of interpretation comes through the multi-grid tube I2I8. This tube is normally biased to cut off. However, through the additional section of the timestick leading to the pulse it previously referred to, a positive pulse is impressed on the triodes I35I and I352 connected in tandem to give requisite amplification without change of polarity of the pulse. Very shortly after the termination of the amplitude code there arrives then on one grid of tube I2I8 a positive pulse of suflicient value to enable the tube I2 I8 for the duration of the pulse n. During this interval then there appears in the output circuit of I2I8 a pulse determined by the magnitude and polarity of the voltage over I2II, which latter is proportional to the amplitude and polarity of the complex wave sample.

These pulses will arrive in succession, one for each sample of the complex wave. By means of the low-pass filter I2I9 the undesired high frequency components may be removed and the resulting wave passing to terminal equipment and receiver will be a reproduction of high fidelity of the original complex wave at the transmitting station.

In this specification my invention has been described in terms of very specific apparatus. This has been for the purpose of clarity but it will be apparent to those skilled in the art that many substantial variations may be made in the system without departing from the spirit of the invention. For example, Fig. 12 shows a specific arrangement for control of the conductances, which control comprises the diode I25I and the triode I251. A modification of this control circuit is shown in Fig. 14 in which the triode is replaced by a second diode I253. The resetting pulse coming in over the transformer I252 will discharge the condenser I254 as before. A pulse from the pulse generator coming over transformer I256 is so poled as to render diode I253 conducting if that is the only pulse arriving. This permits the condenser I254 to be charged and the introduction of conductance, this corresponding to an off signal. If an on signal arrives it is transferred through transformer I258 to the diode I253 but the transformer is so poled that its pulse will oppose the simultaneously arriving pulse over transformer I256. Consequently, diode I253 will not be rendered conducting, the condenser I254 will not receive a charge and the corresponding conductance .will not be introduced. Many other variations should also be borne in mind, for example, the T channel of pulses may be omitted with corresponding simplifications, although with some loss of control. Also the local oscillator at the receiving station may be on all the time instead of being triggered on occasionally, for even if left on it is ineffective at the terminal apparatus unless and until the tube I2I8 has been enabled by the pulse coming from n. Still further it will be evident that the modulation and demodulation features of the transmitter station may be omitted, the sample signals out of tube 82B going directly to the input of tube 835 and directly from tube 901 to the resistor 90?, or its equivalent, without the intermediation of the demodulator in the homodyne unit IV. This would mean also the omission of the oscillator II. Corresponding alterations could be made at the receiving station in connection with its local oscillator. In general, however, such omissions or simplifications would lead to sacrifice in operation or quality and it would be a matter of engineering judgment as to how far one may carry out such simplifications.

This invention is related to my inventions dcscrlbed and claimed in my copending applications Ser. No. 592,961, filed May 10, 1945, and Ser. No. 603,989, filed July 9, 1945. It is the .more closely related to the latter application and difiers from it in that whereas in the system of that application conductances are added in parallel across the load circuit of pentode 835 until the residual voltage available across the conductances in parallel becomes less than a certain small reference level, in this application the introduction of conductances in parallel builds up a voltage 6 which is compared with and made equal to the signal voltage. Also in that system, the sum of the conductances G1 G1: in the low conductance position or state should be small compared with the smallest conductance step. In the present disclosure this restriction is not necessary because in that state the voltage e across R is balanced by the voltage in the upper portion of 9 l8 and I find that certain advantages accrue thereby.

What is claimed is:

In a communication system in which the instantaneous amplitude of a complex wave is sampled at frequently recurrent intervals and then represented by a series of pulses of two different signaling conditions, apparatus for obtaining samples of the instantaneous amplitude of the complex wave, means for measuring each of said samples which comprises means for synthesizing a reference voltage by successively smaller and smaller steps, apparatus for comparing the synthesized voltage and the sample, and means for controlling the signaling condition of each of said pulses by said comparing means.

2. In a system for transmitting information on the shape of a signal from a transmitting to a receiving station, a transmitting station comprising means for sampling a signal Wave periodically and storing on a condenser a potential proportional to the sample amplitude, an amplifier tube the input of which is connected across the storage condenser, a network in the load circuit of said tube including two similar detectors connected to receive the tube output in series, and means for increasing the current in one de-- tector and decreasing the current in the other in proportion to the sample amplitude.

3. In a system for transmitting information on the shape of the signal from a transmitting to a receiving station, a transmitting station comprising means for sampling a signal wave periodically and storing on a condenser a potential proportional to the sample amplitude, means for impressing a voltage proportional to the condenser potential on a network comprising a pair of balanced impedances, and a circuit associated with the network and connected to a local source of current and adapted to unbalance the impedances, the voltage in said associated circuit being made proportional to the sample amplitude and serving to reduce the current in one impedance to zero.

4. In a system for transmitting information on the shape of the signal, a transmitting station comprising means for intermittently sampling a signal wave periodically and developing a potential difierence proportional to each sample amplitude, a source of local oscillations, at modula tor controlled by the local oscillator and the sample voltage to yield a modulated wave of local oscillator frequency, a network comprising a pair of balanced detectors and connected to receive the modulated Wave, and a circuit associated with the network and connected to the source of local oscillations and adapted to unbalance the detectors, the voltage in said associated circuit being made proportional to the sample amplitude and serving to reduce the current in one detector to zero.

5. The combination of claim 1 characterized by means for equalizing the sample voltage with the voltage due to said synthesized voltage.

6. The combination of claim 1 characterized by the fact that the means for the step-by-step adjustment comprises a plurality of conductances connected in parallel and each normally possessing a minimum conductance but adapted to be brought to a larger preassigned conductance, and means for compensating the initial minimum conductance whereby the added conductance is proportional to the sample.

7. The combination of claim 4 characterized by the fact that the effective voltage in the associated circuit is made equal to the sample amplitude voltage by means of a plurality of conductances to be introduced or not introduced step by step in the branch circuit, and that there is present a pulse generator and means controlled to build up the conductances step by step by the coordination of the signal amplitude with pulses from the pulse generator until the output from one of the detectors is substantially zero.

8. The combination of claim 4 characterized by the fact that the effective voltage in the associated circuit is made equal to the sample amplitude voltage by means of a plurality of conductances to be introduced or not introduced step by step in the branch circuit, and that there is present a pulse generator and means controlled to build up the conductances step by step by the coordination of the signal amplitude with pulses from the pulse generator until the output from one of the detectors is substantially zero, the successive steps of conductance to be introduced or not introduced being in a decreasing order of magnitude the steps being multiples of the smallest and final step to be taken.

9. A system for transmitting information on the shape of a signal wave comprising a transmitting station and a receiving station the transmitting station comprising a circuit for periodically sampling the amplitude of the wave and for deriving a carrier frequency wave proportional to the said sample amplitude, a pulse gen erator giving rise to a plurality of cycles of pulses, one cycle for each sample, one pulse in the cycle detecting the polarity of the sample, a controllable conductance to be introduced in series with a source of voltage and a resistor, the current through the resistor being substantially proportional to the conductance, a plurality of the pulses in the cycle operating in coordination with the sample amplitude to introduce conductances 23 step by step from larger to smaller steps on an additive basis until the voltage drop across the resistor is equal to the sample amplitude.

10. A system for transmitting information on the shape of the signal Wave from a transillitter to a receiver station the transmitter station comprising a circuit for periodically sampling the amplitude of the wave, a current source adapted to deliver a current proportional to the sample amplitude and constant for the duration of the sampling interval, a circuit for transferring the said current to a polarity and amplitude detecting circuit, a plurality of n conductances adapted to be connected in parallel with each other and in series with a resistor and a current source, a control circuit for each conductance, and a pulse generator to generate cycles of pulses, one pulse in the cycle serving as a marker pulse and as timing the sampling of the signal Wave, another serving as a polarity pulse to cooperate in testing the polarity of the sample amplitude, the remaining pulses operating successively through the control circuits of the conductances in cooperation with the voltage drop over the resistor to introduce one or more conductance steps until the resistor drop is equal to the sample amplitude within an arbitrary small value.

11. A combination of claim characterized by means to compensate for the resistor drop due to the conductances when in their minimum conductance state.

12. The combination of claim 10 characterized by a circuit associated with each control circuit to transmit one condition of signal if its conductance is left in and a different condition of signal if it is removed, the n conductances being graded in size from the smallest of value Go, the largest being first tested for introduction, the system being so operated that the sum of the conductances introduced and left in at the end of the cycle is proportional to the sample amplitude.

13. The combination of claim 10 characterized by a circuit associated with each control circuit to transmit an off signal if its conductance is left in and an on signal if it is removed, the n conductances being graded in size from the smallest of value G0, the largest being first tested for introduction, the system being so operated that the sum of the conductances introduced and left in at the end of the cycle is proportional to the sample amplitude, and further characterized by the fact that the wave sample is first modulated against a carrier frequency whereby the constant current referred to is of carrier frequency and the current source for the resistor and conductances is of the same frequency.

14. In a circuit for measurement of the polarity and the magnitude of an unknown alternating current Voltage, a network including a pair of balanced detectors in series with the voltage source, a branch circuit supplied with current from an independent source of the same frequency as said unknown alternating current voltage, the voltage due thereto in an impedance tending to unbalance the detectors in a direction depending on the polarity of the source to be measured, means for adjusting the current through said impedance until the voltage across it is equal to the unknown voltage, indicated by the reduction of the currentin one detector to zero.

15. The combination of claim 14 characterized by the fact that the means for adjusting the current comprises a plurality of conductances adapted to be connected in parallel in such amount as to be itself proportional to the unknown voltage.

16. In a circuit for the measurement of the polarity and the magnitude of an unknown voltage from an alternating current source, a pair of balanced homodyne detectors in series with the voltage source, a branch, circuit supplied with current from a source of the same frequency as said unknown voltage and of constant voltage, the voltage'due to said current in a resistor tending to unbalance the detectors in a direction depending on the polarity of the source to be measured and to yield rectified voltage of unequal value in the homodyne detectors; means for ad.- justing the current through said resistor until the voltage across it is equal to the unknown voltage, indicated by the reduction of the output of one homodyne detector to zero.

JOHN R; PIERCE.

REFERENCES CITED The following references file of this patent:

UNITED STATES PATENTS are of record in the 

